1. Field of the Invention
The present invention relates generally to the field of mechanical blowers used in industrial applications, and more particularly relates to medium speed high pressure multistage centrifugal blowers.
2. Description of the Prior Art
Centrifugal blowers are commonly used in many industrial applications. In blower terminology, 15 to 25 pound per square inch gage (psig) is generally considered relatively high pressure compared to the conventional medium pressure of 5–15 psig. Examples of common industrial applications of high-pressure capability blowers include: deep tank aeration in modern wastewater treatment plant, supplying air as an oxygen source to the combustion processes, boosting pressure of various gases, and vacuum applications as power source.
However, the pressure in 15–25 psig range is difficult to obtain in conventional direct drive multistage centrifugal blowers. This is mainly due to the low impeller rotational speed, the limitation of number of stages by rotor critical speed, and excessive leakage recycled through impeller eye which causes high discharge temperature.
As a result, many multistage centrifugal blowers are of the medium pressure type, typically having an outlet pressure between 5 to 15 psig when the inlet is at about the atmospheric pressure (blowers below 5 psig are often considered as fans, and blowers above 25 psig are often considered as compressors).
Referring to FIG. 1, there is shown at 10 a typical type of conventional centrifugal direct drive multi-stage (DDMS) blowers. DDMS centrifugal blowers are simple in construction and need little maintenance. They are reliable and low noise because of the low speed and direct drive arrangement.
As shown in FIG. 1, the conventional DDMS blower 10 has several impellers 12 arranged in series on a common shaft 14 supported by two rolling element bearings 16 at each end. Each stage consists of one of the impellers 12 mounted on the shaft 14, and a diffuser 18 followed by a return channel 20. An alternative current (AC) induction motor is typically used to directly drive the blower through a direct drive shaft 22 at 3,000 revolutions per minute (RPM) with 50 Hz power supply (or 3,600 RPM with 50 Hz power supply).
Usually several such stages are needed to obtain the required pressure, and the more stages, the higher the resulting pressure. In conventional DDMS blowers, the vertically split casing style is used to accommodate the requirement of multiple stages so that additional stages can be easily added to obtain higher pressure. However, the maximum number of stages is limited by the rotor's lateral critical speed. Since rolling element bearings are used to simplify the construction, operating speed must be 15–20% below the first critical. For this reason, aluminum impellers are used to lighten up the rotor so that more stages can be stacked to maximize the pressure gain.
Inside each stage, the compression process starts with the rotating impeller 12 where air is accelerated and pressure is raised proportional to the impeller tip velocity square. The air is then discharged into the diffuser 18 where the high-kinetically energized air is further converted into pressure. The return channel 20 then takes the decelerated air, de-swirls and guides it back to the inlet of the next stage. This process will be repeated in the following stages until the pressure reaches the desired level. However, the gas temperature is also raised in this compression process.
Because of the rising pressure across the successive stages, seals are required between the rotating impeller eye and the stationary casing to minimize the leakage losses for each stage. The effectiveness of these seals are therefore directly related to the blower efficiency and discharge temperature.
Referring to FIG. 2, there is shown the most common and economical type of impeller eye seal used in a conventional DDMS 10. As shown in FIG. 2, the impeller eye 24 is the straight labyrinth type. The labyrinth could be on the rotating impeller 12 or on stationary casing 26. The conventional impeller eye seal 28 is arranged in radial direction, where close clearances are kept between the tip of the labyrinth and opposing surface in radial direction. To minimize the leakage for a higher efficiency, especially when flow rate is relatively low, the clearance are kept as small as possible. However, the requirement of very close clearance often increases the manufacturing cost and endangers the machine because any contact with the impeller 12 will result in a machine seizure.
Additional problems exist when DDMS centrifugal blowers are used with 50 Hz power supply which is used in many countries around the world.
For example, if a blower speed is reduced by 20% (for example from 3,600 RPM with 60 Hz power supply to 3,000 RPM with 50 Hz power supply), about 20% of flow and 44% of the pressure would be lost for the same size blower. To compensate the pressure loss while maintaining direct drive arrangement, more stages have to be added. This is not only expensive but also limited by rotor lateral critical speed. Alternatively, external gearbox needs to be added to increase the blower speed. However, this arrangement is bulky in size, complicates the machine setup and maintenance, and increases the capital cost.
Another potential problem of the DDMS used in a high pressure application is caused by the commonly used radial eye seals. Since the main blower parts are vertically split in design and are interlocked together in assembly, there will be accumulated tolerance and clearance in radial direction that reduce the design clearance, which potentially may cause impeller eye seal to rub the casing which in turn causes seizure of the blower. At the same time, the thermal expansion differential between aluminum impeller and cast iron casing would further reduce the radial clearance because aluminum expands twice as much as the cast iron. This effect become worse when higher pressure is attempted because it is always accompanied by higher temperature rise that in turn causes more thermal expansion differential.
Partially to address these limitations, another type of centrifugal blower, high speed integral gear single stage (IGSS) centrifugal blowers have been developed in the past 20 years and became quite popular, especially in countries with 50 Hz power supply.
Referring to FIG. 3, there is shown at 30 a typical IGSS centrifugal blower.
As shown in FIG. 3, the IGSS centrifugal blower 30 has a single impeller 32 overhung on the high-speed pinion shaft 34 of an integral gearbox 36 that is in turn driven by an standard AC motor (not shown). To get the required pressure in a single stage compression, the impeller 32 is rotating at very high speed, typically ranges between 10,000 to 100,000 RPM. The air flow and pressure can be adjusted by changing the gear ratio.
The problems of DDMS blowers are avoided for IGSS by selecting a higher speed ratio gear set without changing the blower. The IGSS blowers are more compact and lightweight, and can also be fitted optionally with inlet guide vane (IGV) and variable diffuser vanes (VDV) to enhance the off-design-point performances.
However, the high impeller speed has to be accommodated by high-speed technologies and ultra-precision manufacturing methods.
For example, the high cost hydrodynamic bearings 40 are often needed instead of the rolling element bearings. In addition, the more costly and complicated forced oil lubrication system 42 are used instead of the simple oil splash system 44 (in FIG. 1). The high speed impeller 32 is made of high strength 5-axis milled or welded steel instead of low cost cast aluminum.
The high-speed IGSS blower 30 also generates much higher noise than a low speed DDMS 10, primarily due to the higher impeller blade loading and tip speed. The noise is of the high frequency, thus very annoying or potentially damaging to its operators nearby. For such higher level noise, the applicable regulations and standards often require a sound enclosure, which further adds to the cost and increases the difficulty in machine maintenance.
It is always desirable to provide a new design and construction of high pressure centrifugal blowers that can achieve high pressure rise and pressure ratio while overcome the problems existed in conventional centrifugal blowers.